1. Field of the Invention
The present invention relates to a global positioning system for determining the position of a mobile object such as an automobile in a global geometrical region.
2. Description of the Prior Art
There are known global positioning systems (GPS) for determining the position of a mobile object in a global geometrical region. Typically, a global positioning system includes a receiver for receiving radio waves transmitted from three or more GPS satellites. The global positioning system determines the position of a reception point where the receivers have received the radio waves, based on quasi-distance data between the GPS satellites and the reception point, including a time offset of the receiver, and positional data of the GPS satellites.
For two-dimensionally determining the position of the mobile object, i.e., determining the latitude and longitude of the mobile object, it is necessary to measure the quasi-distance data with respect to the three GPS satellites. The three data are required because there are mathematically three unknown quantities, i.e., the two-dimensional positions of the mobile object and a deviation or error between the clock of the GPS satellites and the clock of the receiver, or the time offset of the receiver.
If the three-dimensional position (including the altitude) of a mobile object is to be determined by a global positioning system, the global positioning system needs measurement of data with respect to four GPS satellite as there are four unknown quantities involved. Actually, while such a global positioning system associated with a mobile body such as a running automobile is in operation, one of the four GPS satellites may possibly be shielded by an obstacle such as a building, a structure, a tree, or the like near the receiver, and the receiver may be able to receive radio waves from only the remaining three GPS satellites. In such an occasion, the global positioning system can determine the position of the mobile object only two-dimensionally, and produces latitude and longitude data as its output data, but not altitude data.
One conventional global positioning system operates to solve the above problem as follows: When the global positioning system is forcibly shifted from the three-dimensional position measurement mode to the two-dimensional position measurement mode due to an obstacle, as described above, the global positioning system maintains the altitude data obtained in a previous cycle in the three-dimensional position measurement mode, and employs the maintained previous altitude data to produce approximate three-dimensional position data as output data.
FIG. 1 of the accompanying drawings shows such a conventional global positioning system.
First, a controller of the global positioning system determines whether the position of a mobile object can be determined three-dimensionally in a step S20. If the position of the mobile object can be determined three-dimensionally, i.e., if the global positioning system is receiving radio waves from four GPS satellites, then the global positioning system calculates the present position of the mobile object from the received radio waves in a step S21, and produces latitude data X.sub.1, longitude data Y.sub.1, and altitude data Z.sub.1 as output data in a step S22. Then, control goes to an end. If the position of the mobile object cannot be determined three-dimensionally in the step S20, then the controller of the global positioning system determines whether the position of the mobile object can be determined two-dimensionally in a step S23. If the position of the mobile object can be determined two-dimensionally in the step S23, i.e., if the global positioning system is receiving radio waves from three of the four GPS satellites, then the global positioning system calculates latitude data X.sub.2 and longitude data Y.sub.2 in a step S24. Thereafter, the global positioning data produces the latitude data X.sub.2, longitude data Y.sub.2, and the previously calculated altitude data Z.sub.1 as quasi-data in a step S25. Then, control is ended. If the position of the mobile object cannot be determined two-dimensionally in the step S23, the global positioning system produces no position data in a step S26, and control comes to an end.
With the above conventional global positioning system, however, since it employs altitude data obtained in the previous cycles of the three-dimensional position measurement mode when the three-dimensional position measurement mode is disabled in a present cycle, the output data suffer a relatively large error when the altitude of the mobile object varies largely as when the mobile object runs uphill.